The present invention relates to a mechanism and method for holding a substrate, such as a liquid crystal display (hereinafter referred to as "LCD") substrate or semiconductor wafer, on a substrate stage (susceptor, etc.) in a substrate treatment apparatus for subjecting the substrate to treatments, such as film deposition, etching, ashing, etc.
Plasma treatment apparatuses are frequently used to manufacture semiconductor products and the like that have recently been becoming higher in density and finer. These apparatuses carry out film deposition, etching, ashing, and other treatments with use of plasma. It is feared, however, that the etching, ashing, or device characteristics of the plasma treatment apparatuses of this type are adversely affected by heat that is produced as the plasma is generated. In etching or ashing a to-be-treated film, such as a semiconductor film, metal film, or resist film, on an LCD substrate with use of plasma, in a process for manufacturing an active-matrix LCD, for example, the etching, ashing, or device characteristics may possibly receive bad influences from the heat attributable to the plasma generation or heat that is produced by a chemical reaction between the plasma and the to-be-treated film.
In order to reduce these bad influences of heat, an attempt has recently been made to improve the rate of heat transfer between the substrate and the substrate stage through the medium of helium gas so that heat can be efficiently released from the substrate and transferred to the substrate stage that has a cooling function.
FIG. 4 shows a conventional parallel-plate plasma etching apparatus 10. As shown in FIG. 4, the apparatus 10 comprises a treatment vessel 24 that forms a treatment chamber 12. The chamber 12 contains therein a substrate stage 14 as a lower electrode, on which an LCD substrate P, for example, is placed, and an upper electrode 16 that faces the stage 14 from above. A high-frequency power source 28 is connected to the upper electrode 16 through a matching device 26, whereby high-frequency power is applied to the electrode 16 so that plasma is generated between the electrode 16 and the substrate stage 14 as the lower electrode.
A refrigerant circuit 18 for use as temperature regulating means is provided inside the substrate stage 14. The stage 14 can be kept at a given temperature by circulating a refrigerant from a refrigerant source (not shown) through the refrigerant circuit 18. A clamp 30 is provided on the upper surface of the substrate stage 14. The clamp 30 presses the peripheral edge portion of the LCD substrate P on the stage 14, thereby mechanically holding the substrate P on the stage 14. The substrate stage 14 is provided with a large number of gas supply holes 34 opening in its upper surface that carries the LCD substrate P thereon. The holes 34 are connected to a gas supply pipe 32, which is connected to a helium gas source (not shown).
An exhaust pipe 22, which is connected to a vacuum pump (not shown), is attached to the lower part of a side wall of the treatment vessel 24, whereby the treatment chamber 12 can be evacuated to a predetermined vacuum pressure. A top wall of the vessel 24 is provided with a gas inlet pipe 20 through which a specified treatment gas is introduced into the chamber 12.
In the plasma etching apparatus 10 constructed in this manner, the specified treatment gas is introduced into the treatment chamber 12 through the gas inlet pipe 20 after the LCD substrate P is placed on the substrate stage 14. At this time, the chamber 12 is evacuated through the exhaust pipe 22, whereupon it is kept at the predetermined vacuum pressure. In this state, the high-frequency power from the high-frequency power source 28 is applied to the upper electrode 16 through the matching device 26, so that plasma is generated between the electrode 16 and the substrate stage 14 as the lower electrode. Thus, the to-be-treated surface of the LCD substrate P is etched by the agency of radicals, ions, etc. that are produced when the plasma is generated.
During this etching process, the interior of the treatment chamber 12 is kept at a vacuum pressure of, for example, tens to hundreds of mTorr. If the LCD substrate P is simply placed on the substrate stage 14 under this low pressure, the adhesion between the substrate P and the stage 14 is too low to transfer the heat from the substrate P satisfactorily to the stage 14. As a result, the cooling function of the stage 14 cannot be fulfilled, so that the temperature of the substrate P inevitably increases to an excessive extent. In order to enhance the adhesion between the substrate P and the stage 14, thereby restraining the substrate temperature from rising, therefore, the peripheral edge portion of the substrate P is pressed against the stage 14 by means of the clamp 30, and helium gas at, for example, several Torr is introduced into the region between the substrate P and the stage 14 through the gas supply pipe 32 and the gas supply holes 34. The helium gas between the substrate P and the stage 14 improves the rate of heat transfer between the two, and allows the heat from the substrate P to transfer efficiently to the stage 14. Thus, the temperature of the LCD substrate P can be prevented from increasing excessively.
When placing a semiconductor wafer on the substrate stage 14, in general, the wafer is attracted electrostatically to the stage 14 by means of an electrostatic chuck that is capable of all-over attraction. When the LCD substrate P is placed on the stage 14, however, the electrostatic chuck cannot serve as holding means, since the substrate P is formed of electrically insulated glass. As mentioned before, therefore, the substrate P is mechanically held on the stage 14 in a manner such that its peripheral edge portion is pressed by means of the clamp 30. If helium gas at several Torr, for example, is introduced into the region between the substrate P and the stage 14 in this state, however, the central portion of the substrate P is urged upward by a pressure equivalent to the difference between the pressure from the helium gas, which acts on the inner surface of the substrate P, and the pressure (tens to hundreds of mTorr) in the treatment chamber 12, which acts on the outer surface of the substrate. Thereupon, the central portion of the substrate P swells and separates from the stage 14. Even though the helium gas is introduced expressly to improve the rate of heat transfer between the substrate P and the stage 14, therefore, heat transfer at the central portion of the substrate P is not satisfactory, so that the temperature of the substrate P cannot be restrained from rising. If the LCD substrate P has a substantial thickness (e.g., about 1.1 mm), in this case, its deflection attributable to the differential pressure is so small that the problem of heat transfer failure exerts no bad influences on plasma treatment. If the substrate P is relatively thin (e.g., about 0.7 mm thick), however, the differential pressure causes it to bend substantially, so that the rate of heat transfer at the central portion of the substrate P is worsened. Thus, there is a possibility of the etching characteristics being adversely affected. This arouses a serious problem in these days of development of thinner, larger LCD substrates. Naturally, the substrate stage 14 may be shaped in consideration of the deflection of the LCD substrate P. If this is done, however, the treatment accuracy and costs involve problems, and it is very difficult to put this substrate stage to practical use.